Many types of wet particulate materials or industrials products must have their moisture content reduced in order to render the materials commercially useful or capable of economic transportation and storage, or to prevent their deterioration. The type of wet materials which may require such drying include industrial waste or sewage sludge, pulp sludge, various slurries, grains, cereals, organic and inorganic fibres and pulps and chemical waste. Given increasingly stringent environmental standards and regulations, the drying, or removal of a portion of the moisture, of the wet materials is preferably accomplished in a relatively environmentally safe manner by reducing the pollutants released to the environment as a result of the drying process.
In the case of grain or cereal crops, rain or damp weather at harvest time may prevent natural drying in the field and result in deterioration and possible rotting of the crops. The commercial value of the crop may be preserved by quickly drying the crop to allow for its safe transportation and storage. However, the crop must be dried without damaging the grain or cereal. In addition, the moisture removed from the crop is preferably recycled and returned to the environment without the addition of any harmful pollutants to the environment.
Similar drying processes may also be required for the byproducts or wastes produced by industrial processes. These industrial byproducts often contain chemicals or other harmful materials which tend to pollute the environment, but which must be disposed of in some fashion. Such industrial byproducts are often in the form of fine inorganic and organic particles suspended or otherwise diffused in water and are known as sludges or slurries. Examples of sources of such sludges or slurries include pulp and paper mills, sewage treatment plants, chemical plants and other food processing or industrial operations. In addition to disposal problems, these industrial byproducts are often difficult and costly to handle and transport due to their high liquid or moisture content. Thus dehydration of these materials, by removing the excess moisture, is required. Preferably, the dehydration is conducted in a manner rendering both the removed moisture and the dehydrated material relatively environmentally friendly as compared to the initial industrial byproduct. In addition, in many cases, the dehydration of the industrial byproducts will sterilize the harmful or toxic substances contained in them and render them commercially valuable as, for example, a fertilizer.
Drying of these types of wet materials, and removal of their excess moisture, may be accomplished mechanically through the use of belt presses, centrifuges, filters and holding ponds for settling and separation over time. Mechanical drying typically removes only a limited portion of the moisture content and does not affect any pollutants contained in the resulting processed materials.
Another method previously utilized for drying these wet materials involves indirect heating using furnaces, kilns, burners, ovens, and the like. Indirect heating typically involves blowing hot air produced by the furnace, or other indirect heating apparatus, across the wet material. Although the volume of moisture contained in the wet material is reduced, many of the pollutants are released into the vapour produced by the heat which may in turn become mixed with the flue gases and released to the atmosphere. In addition, the furnace is typically heated by means of combustion resulting in the further release of polluted flue gases into the atmosphere. Finally, the temperature of the wet material during the drying process is often difficult to control and may result in burning of the dehydrated material, rendering it of limited commercial value. As a result, the dehydrated material may need to be incinerated.
A further method for drying the wet materials involves using a combination of radiant heat and hot air produced through the combustion of heating oil or gas. Flame burners are used to produce the radiant heat. The exhaust gases from the flame burners produce the hot air for the indirect heating. The hot air is not in direct contact with the material, but rather, is blown through a casing in a trough conveying the material and through the hollow axles of auger shafts moving the material through the trough. One example showing this combination of radiant heat and combustion gases to dry sewage sludge is shown in PCT Application No. PCT/CA90/00074 to Schmidt et. al. published Sep. 7, 1990.
Several disadvantages may arise from applying an indirect heat to the wet material through the circulation of combustion exhaust gases. The temperature of the material through the entire drying process and apparatus may be difficult to control. In particular, the temperature of the exhaust gases from the flame burners, distributed throughout the apparatus, may be constant regardless of the moisture level in the material at any particular point in the apparatus and regardless of the particular stage of dehydration of the material. It is therefore difficult to control the heat gradient as the material is dehydrated, which may result in burning. As well, difficulty in controlling the temperature of the exhaust gases may result in overheating of the apparatus and burning of the material.
The use of exhaust gases to indirectly heat the material, and attempts to better control the temperature of the exhaust gases, have resulted in relatively complicated apparatuses incorporating equipment such as: fire boxes for combustion; jacketed pipes, troughs and augers to convey the hot exhaust gases; and complicated water cooling systems involving water injection apparatus for reducing the temperature of the hot exhaust gases circulated through the drying apparatus. The use of these types of relatively complicated equipment may result in greater maintenance and repair problems.
Finally, combustion of oil and gas produces exhaust gases containing pollutants which may be harmful to the environment and which may require treatment prior to their discharge. For example, the exhaust gases may contain sulfuric acid. In addition to being a pollutant, sulfuric acid tends to corrode any surface it comes into contact with. Therefore, corrosion may become a problem within the areas of the apparatus circulating the exhaust gases, such as the jacketed pipes, troughs and augers.
There is therefore a need in the industry for a method and an apparatus for removing moisture from a wet material in a relatively environmentally safe manner, without damaging the material, by applying heat in a relatively precisely controlled manner. In addition, the apparatus should preferably be relatively simple to construct and maintain as compared to previously known drying apparatuses.